Electroless copper coating process for chromium metal powders

ABSTRACT

Disclosed herein are methods for electrolessly coating copper onto a chromium metal powder, the method comprising adding a nickel-coated chromium powder to an aqueous electroless copper plating bath comprising a source of copper cations, a copper-cation complexing agent, a copper-cation reducing agent, and a first base, thereby forming a copper-coated chromium metal powder. Also disclosed are copper-coated chromium metal powders prepared by a disclosed method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.63/187,601, filed May 12, 2021, the entirety of which is incorporatedinto this application by reference.

BACKGROUND

Composite and coated metal powders are increasingly being used inthermal spray and high-velocity spray applications to maintain elementalratios and to help prevent segregation due to differences in particlemorphology and densities between the different metal components.

Copper-chromium metal composites are typically made by physically mixingthe individual metal powders followed by pressing and sintering. Due tothe differences between copper and chromium, inhomogeneities in theresulting composite are the result. Making the composite material fromcoated metal powders can vastly improve the homogeneity of the finalmaterial.

Chromium metal powder is difficult to use as a substrate for electrolessdeposition due to its passivating oxide coating. There is a need in theart for a process in which electroless, or autocatalytic copper or anyother metal coatings can be deposited on chromium metal powder. Theseneeds and other needs are satisfied by the methods of the presentdisclosure.

SUMMARY

In accordance with the purpose(s) of the invention, as embodied andbroadly described herein, this disclosure, in one aspect, relates to anelectroless copper coating process for chromium metal powders, chromiummetal powders prepared by the disclosed process, and uses thereof.

In one aspect, disclosed is a method for electrolessly coating copperonto a chromium metal powder, the method comprising adding anickel-coated chromium powder to an aqueous electroless copper platingbath comprising a source of copper cations, a copper-cation complexingagent, a copper-cation reducing agent, and a first base, thereby forminga copper-coated chromium metal powder.

In a further aspect, disclosed are copper-coated chromium metal powderprepared by a disclosed method.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, which are incorporated in and constitute apart of this specification, illustrate several aspects and together withthe description serve to explain the principles of the invention.

FIG. 1 is an SEM image of 0.44 wt. % nickel coated chromium metalpowder.

FIG. 2 is an SEM image of 37 wt. % Cu coated chromium metal powder.

FIG. 3 is a cross-sectional SEM image of 8% Ni, 37% Cu coated chromiummetal powder showing conformal Ni/Cu coating of chromium metalparticles.

Additional aspects of the invention will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or can be learned by practice of the invention. Theadvantages of the invention will be realized and attained by means ofthe elements and combinations particularly pointed out in the appendedclaims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the invention, as claimed.

DESCRIPTION

The present invention can be understood more readily by reference to thefollowing detailed description of the invention and the Examples.

A. Definitions

When the term “about” precedes a numerical value, the numerical valuecan vary within ±10% unless specified otherwise.

As used herein, the terms “optional” or “optionally” means that thesubsequently described event or circumstance can or can not occur, andthat the description includes instances where said event or circumstanceoccurs and instances where it does not.

As used herein, the term “by weight,” when used in conjunction with acomponent, unless specially stated to the contrary is based on the totalweight of the formulation or composition in which the component isincluded. For example, if a particular element or component in acomposition or article is said to have 8% by weight, it is understoodthat this percentage is in relation to a total compositional percentageof 100%.

“Chromium powder” refers to a powder having at least 95% Cr metal, thebalance weight of the powder being any trace metals. In some aspects,the “chromium powder” has at least 99% (e.g., at least 99.95%) Cr metal.“Chromium powder” does not refer to any non-metallic particles such asdiamond that have been coated with chromium, such as described in Niaziet al., “Parameters optimization of electroless deposition of Cu onCr-coated diamond,” Trans. Nonferrous Met. Soc. China 24, 136-145(2014). In further aspects, “chromium powder” refers to a powder that isfree of chromium carbide (Cr₃C₂) and in some aspects, free of anychromium ion In some aspects, the “chromium powder” has an averageparticle size as measured according to techniques known in the artranging from 10-150 microns.

A “roughening treatment” refers to any known method of roughening thesurface of a metal powder, including a roughening treatment with acid(e.g., nitric acid, sulfuric acid, hydrofluoric acid, combinations ofthese acids) optionally in combination with other agents such ashydrogen peroxide. The “roughening treatment” can include any thosemethods described in US 2019/0240729, which is incorporated by referencefor its teaching of roughening treatments. “Roughening treatment” alsoincludes methods described in Luo, et al., “Preparation andcharacterization of Ni-coated Cr₃C₂ powder by room temperatureultrasonic-assisted electroless plating,” Ceramics International 36,1989-1992 (2010), including coarsening treatment prior to plating byimmersing a metal powder into an aqueous solution of hydrofluoric acid,nitric acid, and ammonium fluoride. An unexpected advantage of thedisclosed methods is that in some aspects, the method does not requireany roughening treatment prior to either metal coating step.

B. Electroless Plating Method

The disclosed method enables the electroless deposition of copper onto asubstrate, e.g., chromium, that is not active towards electroless copperdeposition. Additionally, an advantage of the disclosed method is thatthe amount of nickel required to initiate the copper plating isrelatively small, so as not to affect the conductivity of the finalcopper coated chromium.

In one aspect, the method for electrolessly coating copper onto achromium metal powder comprises adding a nickel-coated chromium powderto an aqueous electroless copper plating bath comprising a source ofcopper cations, a copper-cation complexing agent, a copper-cationreducing agent, and a first base, to thereby form a copper-coatedchromium metal powder.

According to one aspect, the source of copper cations can be a cupric(Cu(II)) salt. In some aspects, the cupric (Cu(II)) salt can be coppersulfate (CuSO₄), copper chloride (CuCl₂), copper nitrate (Cu(NO₃)₂),copper acetate (Cu(OAc)₂), copper flouride (CuF₂), copper bromide(CuBr₂), copper carbonate (CuCO₃), copper chlorate (Cu(ClO₃)₂), copperphosphate (Cu₃(PO₄)₂), copper perchlorate (Cu(ClO₄)₂), copper sulfide(CuS), copper tetrafluroborate (Cu(BF₄)₂), or a combination thereof.

In one aspect, the copper-cation complexing agent can be a tartratesalt, an alkanol amine, a glycolic acid, ethylenediaminetetraacetic acid(EDTA), or a combination thereof. The alkanol amine, for example, whenpresent, can be a C₁-C₁₄ alkanol amine. Non-limiting examples ofsuitable alkanol amines are ethanolamine, diethanolamine,triethanolamine, N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine), ora combination thereof.

According to one aspect, the first base can be sodium hydroxide, lithiumhydroxide, potassium hydroxide, tetraethylammonium hydroxide, lime, or acombination thereof.

In one aspect, the copper-cation reducing agent can be glyoxylic acid,formaldehyde, dimethylamine borane, borohydride, hypophosphite,hydrazine, sucrose, glucose, dithionite, or a combination thereof.

The method generally involves first preparing the nickel-coated chromiumpowder. The nickel-coated chromium powder can be prepared by a methodcomprising adding chromium powder to an aqueous electroless nickelplating bath comprising a source of nickel cations, a nickel-cationcomplexing agent, a nickel-cation reducing agent, and a second base, tothereby coat the chromium powder with nickel.

According to one aspect, the source of nickel cations for coating thechromium powder with nickel can be a nickel(II) salt. Non-limitingexamples of suitable nickel(II) salts include nickel sulfate (NiSO₄),nickel chloride (NiCl₂), nickel flouride (NiF₂), nickel iodide (NiI₂),nickel bromide (NiBr₂), nickel formate (Ni(CHO₂)₂), nickel acetate(Ni(CH₃CO₂)₂), nickel ammonium sulfate (Ni(NH₄)₂(SO₄)₂, nickel sulfamate(Ni(SO₃NH₂)₂, nickel fluoborate (Ni(BF₄)₂), nickel borate (Ni₃(BO₃)₂),or a hydrate or combination thereof.

In one aspect, the nickel-cation complexing agent can be sodiumpyrophosphate. The second base can be a suitable base for maintaining anadequate pH, e.g., ammonium hydroxide. The nickel-cation reducing agentcan be sodium hypophosphite.

The chromium powder can be coated with nickel in an electroless nickelplating bath. The plating bath can comprise water, e.g., deionizedwater, a suitable nickel-cation complexing agent such as sodiumpyrophosphate. The bath can further comprise the nickel(II) salt, suchas nickel sulfate. Additionally, the bath can comprise a suitablenickel-cation reducing agent such as sodium pyrophosphate and a suitablebase such as ammonium hydroxide.

The nickel plating bath can then be heated to a suitable temperature,e.g., from 50° C. to 80° C. According to one aspect, the nickel platingbath can be heated to a temperature ranging from 60° C. to 70° C., e.g.,65° C. Once the nickel plating bath is heated to a suitable temperature,the bath can be agitated, and chromium metal powder can be added withagitation. The resulting bath can be agitated for a suitable time, e.g.,from 60 to 120 minutes. According to one aspect, the bath can beagitated for about 90 minutes.

After a suitable agitation period, the resulting powder can be dried inan inert atmosphere such as argon. Drying can be accomplished at asuitable temperature, e.g., from 90° C. to about 120° C. According toone aspect, the powder can be dried in an oven at a temperature of about110° C. Drying can be accomplished over a suitable time, e.g., 8-15hours. According to one aspect, sufficient drying can be accomplished inabout 12 hours.

By varying the amount of chromium metal powder added to the bath, thenickel percentage in the powder can be altered. For example, nickelcoatings having from about 0.1% by weight to about 20% by weight ofnickel can be prepared. According to one aspect, nickel coatings havingfrom about 0.2% by weight to about 10% by weight of nickel can beprepared using the disclosed method. For example, powders comprising0.2%, 0.3%, 0.4%, 0.44%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 10% by weightof nickel can be prepared. An example SEM image of a nickel-coatingchromium powder comprising 0.44% by weight of nickel is shown in FIG. 1.

The method generally does not require stabilizing either electrolessbath, e.g., by adjusting the pH to prevent accumulation of a salt ontothe metal powder. This can be accomplished in some aspects by consumingthe entire source of nickel in the first plating step and/or the entiresource of copper in the second plating step. In addition, in someaspects, neither the first electroless plating bath nor the secondelectroless plating bath are re-used, i.e., some aspects of the methodare batch as opposing to continuous process or methods in which one ormore baths is re-used for a subsequent plating method.

To prepare the copper-coat on the nickel-coated chromium powder, anelectroless copper plating method can be used. An electroless copperplating bath can be prepared in a suitable bath. The bath can comprisewater, e.g., deionized water heated to a temperature of from about 35°C. to about 45° C. A suitable second base such as sodium hydroxide canbe added, along with a suitable copper-cation complexing agent such asEDTA. A suitable source of copper cations such as a cupric (Cu(II))salt, e.g., copper sulfate, can be added to the bath, along with thecopper-cation reducing agent such as glyoxylic acid.

Agitation of the electroless copper bath can then be increased, afterwhich a suitable amount of the nickel-coating chromium powder can beadded. After a suitable agitation time, e.g., 15 minutes to 45 minutes,an additional amount of the source of copper cations can be added, e.g.,copper sulfate. An additional amount of the second based, e.g., sodiumhydroxide, and the copper-cation reducing agent, e.g., glyoxylic acid,can be added. The resulting mixture can be allowed to react for asuitable time, e.g., 15 minutes to 45 minutes. According to one aspect,the reaction time can be about 25 minutes. This process can be repeatedas desired, e.g., two additional times.

Once the electroless copper coating process has occurred, the platingbath can be filtered and washed with deionized water. Washing withdeionized water can dissolve any byproducts resulting from thecopper-cation reducing agent. For example, when the copper-cationreducing agent is glyoxylic acid, washing with deionized water sufficesto removed oxalic acid byproduct. An exemplary powder produced accordingto this method is shown in the SEM image of FIG. 2. FIG. 3 shows across-sectional SEM image of a similar copper-coated chromium metalpowdering comprising 8% by weight nickel.

C. Copper-Coated Chromium Metal Powders

Also disclosed are copper-coated chromium metal powders prepared by adisclosed method. According to one aspect, the chromium metal powder cancomprise from 1% to 50% by weight of copper. According to a furtheraspect, the chromium metal powder can comprise from 10% to 40% by weightof copper. In a still further aspect, the chromium metal powder cancomprise from 30% to 40% by weight of copper. In yet a further aspect,the chromium metal powder can comprise from 35% to 40% by weight ofcopper. Additionally, in some aspect, the chromium metal powder cancomprise from 0.2% to 10% by weight of nickel.

D. Aspects

The present invention can be described in various non-limiting aspects,such as the following.

Aspect 1: A method for electrolessly coating copper onto a chromiummetal powder, the method comprising adding a nickel-coated chromiumpowder to an aqueous electroless copper plating bath comprising a sourceof copper cations, a copper-cation complexing agent, a copper-cationreducing agent, and a first base, thereby forming a copper-coatedchromium metal powder.

Aspect 2: The method of Aspect 1, wherein the source of copper cationsis a cupric (Cu(II)) salt.

Aspect 3: The method of Aspect 1 or 2, wherein the cupric (Cu(II)) saltis copper sulfate (CuSO₄), copper chloride (CuCl₂), copper nitrate(Cu(NO₃)₂), copper acetate (Cu(OAc)₂), copper flouride (CuF₂), copperbromide (CuBr₂), copper carbonate (CuCO₃), copper chlorate (Cu(ClO₃)₂),copper phosphate (Cu₃(PO₄)₂), copper perchlorate (Cu(ClO₄)₂), coppersulfide (CuS), copper tetrafluroborate (Cu(BF₄)₂), or a combinationthereof.

Aspect 4: The method of any of Aspects 1-3, wherein the copper-cationcomplexing agent is a tartrate salt, an alkanol amine, a glycolic acid,ethylenediaminetetraacetic acid (EDTA), or a combination thereof.

Aspect 5: The method of any of Aspects 1-4, wherein the alkanol amine,when present, is a C₁-C₁₄ alkanol amine.

Aspect 6: The method of any of Aspects 1-5, wherein the alkanol amine,when present, is ethanolamine, diethanolamine, triethanolamine,N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine), or a combinationthereof.

Aspect 7: The method of any of Aspects 1-6, wherein the first base issodium hydroxide, lithium hydroxide, potassium hydroxide,tetraethylammonium hydroxide, lime, or a combination thereof.

Aspect 8: The method of any of Aspects 1-7, wherein the copper-cationreducing agent is glyoxylic acid, formaldehyde, dimethylamine borane,borohydride, hypophosphite, hydrazine, sucrose, glucose, dithionite, ora combination thereof.

Aspect 9: The method of any of Aspects 1-8, wherein the nickel-coatedchromium powder is prepared by a method comprising adding chromiumpowder to an aqueous electroless nickel plating bath comprising a sourceof nickel cations, a nickel-cation complexing agent, a nickel-cationreducing agent, and a second base, thereby coating the chromium powderwith nickel.

Aspect 10: The method of Aspect 9, wherein the source of nickel cationsis a nickel(II) salt.

Aspect 11: The method of any of Aspects 9 or 10, wherein the nickel(II)salt is nickel sulfate (NiSO₄), nickel chloride (NiCl₂), nickel flouride(NiF₂), nickel iodide (NiI₂), nickel bromide (NiBr₂), nickel formate(Ni(CHO₂)₂), nickel acetate (Ni(CH₃CO₂)₂), nickel ammonium sulfate(Ni(NH₄)₂(SO₄)₂, nickel sulfamate (Ni(SO₃NH₂)₂, nickel fluoborate(Ni(BF₄)₂), nickel borate (Ni₃(BO₃)₂), or a hydrate or combinationthereof.

Aspect 12: The method of any of Aspects 9-11, wherein the nickel-cationcomplexing agent is sodium pyrophosphate.

Aspect 13: The method of any of Aspects 9-12, wherein the second base isammonium hydroxide.

Aspect 14: The method of any of Aspects 9-13, wherein the nickel-cationreducing agent is sodium hypophosphite.

Aspect 15: A copper-coated chromium metal powder prepared by the processof any of Aspects 1-14.

Aspect 16: The metal powder of Aspect 15, comprising from 1% to 50% byweight of copper.

Aspect 17: The metal powder of Aspect 15 or 16, comprising from 10% to40% by weight of copper.

Aspect 18: The metal powder of any of Aspects 15-17, comprising from 30%to 40% by weight of copper.

Aspect 19: The metal powder of any of Aspects 15-18, comprising from 35%to 40% by weight of copper.

Aspect 20: The metal powder of any of Aspects 15-19, comprising from0.2% to 10% by weight of nickel.

E. Examples

The examples described herein are put forth so as to provide those ofordinary skill in the art with a complete disclosure and description ofhow the methods claimed herein are made and evaluated, and are intendedto be purely exemplary of the invention and are not intended to limitthe scope of what the inventors regard as their invention. Efforts havebeen made to ensure accuracy with respect to numbers (e.g., amounts,temperature, etc.), but some errors and deviations should be accountedfor. Unless indicated otherwise, parts are parts by weight, temperatureis in ° C. or is at ambient temperature, and pressure is at or nearatmospheric.

Electroless coatings of copper on chromium metal powder was achieved byfirst depositing a small amount of electroless nickel on the surface ofthe chromium metal powder. An example follows for making a 35-40 wt. %copper coated chromium metal powder.

An electroless nickel plating bath was made in a 20 L glass cellcontaining four baffles and equipped with an overhead stirrer andnitrogen sparging. The plating bath contained 10 L deionized water,132.5 g sodium pyrophosphate (Na₂P207-10H₂0), 67 g Nickel sulfate(NiSO₄-6H₂O), 185.6 g sodium hypophosphite (NaH₂P02-1-120), and 600 mLammonium hydroxide (NH₄OH). After the bath was heated to 65° C., theagitation was increased to 420 RPM and 3 kg of chromium metal powder (45pm) was added. After 90 minutes the bath was filtered, and the powderwashed with water. The powder was dried in an argon atmosphere oven at110° C. for 12 hours.

ICP analysis of the resulting powder is shown in Table 1 and its SEManalysis is shown in FIG. 1. By varying the amount of chromium metalpowder added to the bath, the nickel percentage can be altered. Nickelcoatings from 0.2 wt. % to 10 wt. % can be prepared, for example.

TABLE 1 ICP Analysis of Nickel-Coated Chromium Metal Powder ElementComposition Ni 0.44 wt. % P 490 ppm

This nickel coated chromium metal powder was then coated with copper byfirst preparing an electroless copper plating bath in the same 20 Lcell, though without nitrogen sparging. The electroless copper platingbath contained 10 L hot deionized water (35-45° C.), 450 g sodiumhydroxide (NaOH), 400 g ethylenediamine tetraacetic acid (EDTA), 171 gcopper sulfate (CuSO₄-5H₂0), and 350 mL glyoxylic acid (50%).

The agitation was increased to 500 RPM, after which 260 g of the nickelcoated chromium metal powder was added. After 25 minutes an additional170 g copper sulfate (CuSO₄-5H₂0), 140 g sodium hydroxide, and 170 mLglyoxylic acid were added and allowed to react for 25 minutes. This wasrepeated two additional times, after which the bath was filtered andwashed with copious amounts of deionized water to dissolve the oxalicacid byproduct and wash the copper coated nickel chromium metal powder.

ICP analysis of the coated powder is shown in Table 2. SEM analysis ofthe powder is shown in FIG. 2. FIG. 3 shows a cross-sectional SEM imageof a similar copper coated chromium metal powder containing 8 wt. %nickel.

TABLE 2 ICP Analysis of Copper-Coated Nickel Chromium Metal PowderElement Composition Cu 37 wt. % Ni 0.16 wt. % P <50 ppm

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the scope or spirit of the invention. Otherembodiments of the invention will be apparent to those skilled in theart from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A method for electrolessly coating copper onto achromium metal powder, the method comprising adding a nickel-coatedchromium powder to an aqueous electroless copper plating bath comprisinga source of copper cations, a copper-cation complexing agent, acopper-cation reducing agent, and a first base, thereby forming acopper-coated chromium metal powder; wherein the nickel-coated chromiumpowder is prepared by adding chromium powder to an aqueous electrolessnickel plating bath comprising a source of nickel cations, anickel-cation complexing agent, a nickel-cation reducing agent, and asecond base, thereby coating the chromium powder with nickel.
 2. Themethod of claim 1, wherein the chromium powder has not been subjected toa roughening treatment prior to coating the chromium powder with nickel.3. The method of claim 1, wherein the nickel-coated chromium powder hasnot been subjected to a roughening treatment prior to coating thenickel-coated chromium powder with copper.
 4. The method of claim 1,wherein the source of copper cations is a cupric (Cu(II)) salt.
 5. Themethod of claim 4, wherein the cupric (Cu(II)) salt is copper sulfate(CuSO₄), copper chloride (CuCl₂), copper nitrate (Cu(NO₃)₂), copperacetate (Cu(OAc)₂), copper flouride (CuF₂), copper bromide (CuBr₂),copper carbonate (CuCO₃), copper chlorate (Cu(ClO₃)₂), copper phosphate(Cu₃(PO₄)₂), copper perchlorate (Cu(ClO₄)₂), copper sulfide (CuS),copper tetrafluroborate (Cu(BF₄)₂), or a combination thereof.
 6. Themethod of claim 1, wherein the copper-cation complexing agent is atartrate salt, an alkanol amine, a glycolic acid,ethylenediaminetetraacetic acid (EDTA), or a combination thereof.
 7. Themethod of claim 6, wherein the alkanol amine, when present, is a C₁-C₁₄alkanol amine.
 8. The method of claim 7, wherein the alkanol amine, whenpresent, is ethanolamine, diethanolamine, triethanolamine,N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine), or a combinationthereof.
 9. The method of claim 1, wherein the first base is sodiumhydroxide, lithium hydroxide, potassium hydroxide, tetraethylammoniumhydroxide, lime, or a combination thereof.
 10. The method of claim 1,wherein the copper-cation reducing agent is glyoxylic acid,formaldehyde, dimethylamine borane, borohydride, hypophosphite,hydrazine, sucrose, glucose, dithionite, or a combination thereof. 11.The method of claim 1, wherein the source of nickel cations is anickel(II) salt.
 12. The method of claim 11, wherein the nickel(II) saltis nickel sulfate (NiSO₄), nickel chloride (NiCl₂), nickel flouride(NiF₂), nickel iodide (NiI₂), nickel bromide (NiBr₂), nickel formate(Ni(CHO₂)₂), nickel acetate (Ni(CH₃CO₂)₂), nickel ammonium sulfate(Ni(NH₄)₂(SO₄)₂, nickel sulfamate (Ni(SO₃NH₂)₂, nickel fluoborate(Ni(BF₄)₂), nickel borate (Ni₃(BO₃)₂), or a hydrate or combinationthereof.
 13. The method of claim 1, wherein the nickel-cation complexingagent is sodium pyrophosphate.
 14. The method of claim 1, wherein thesecond base is ammonium hydroxide.
 15. The method of claim 1, whereinthe nickel-cation reducing agent is sodium hypophosphite.
 16. The methodof claim 1, wherein the chromium powder has an average particle sizeranging from 10-150 microns.
 17. The method of claim 1, consistingessentially of the steps recited.
 18. The method of claim 1, consistingof the steps recited.
 19. A copper-coated chromium metal powder preparedby the process of claim
 1. 20. The metal powder of claim 19, comprisingfrom 1% to 50% by weight of copper and 0.2% to 10% by weight of nickel.